Nitrates and nitrites may be present in aqueous wastes for a variety of reasons. For example, processing of nuclear materials generates enormous amounts of aqueous low level radioactive wastes. These aqueous wastes frequently contain large amounts of nitrates and nitrites, usually in the form of sodium nitrate or nitrite.
Most manufacturing industries generate nitrate waste. Nitrates appear on the Clean Water Act's list of 126 priority pollutants published in 40CFR 122, Appendix D. They also appear on the EPA's Toxic Release Inventory (TRI) where they are 18th out of 100 prime pollutants. Industrial nitrate waste generators are forced by the regulations and by the public to reduce or eliminate nitrate discharges to the environment.
Generation of nitrate wastes by various industries is vast. According to the EPA's TRI database, the total production-related nitrate waste amounted to 781 million lb. in 1997. Among the major generators are manufacturers of chemicals and allied products (SIC code 28; 430 million lb.), including industrial organic chemicals (SIC no. 2869), nitrogenous fertilizers (SIC no. 2873), industrial inorganic chemicals (SIC no. 2819), organic fibers (SIC no. 2824) and plastic materials and resins (SIC no. 2821); primary metal industries (SIC code 33; 134 million lb.), including blast furnaces and steel mills (SIC no. 3312); food and kindred products (SIC code 20; 72 million lb.); and electronic & other electric equipment (SIC code 36; 45 million lb.).
Economical and environmentally safe methods of disposing of nitrate and nitrite-containing aqueous waste, including low level nuclear wastes, have been sought for years. Many of the methods involve the addition of clay or Portland cement to the liquid waste, in combination with various other materials, to solidify the waste and prevent leaching of radioactive or other hazardous materials from storage locations. For example, U.S. Pat. No. 3,274,784, describes the solidification and immobilization of radioactive waste solutions by the addition of clay, lime and caustic. U.S. Pat. No. 5,640,704 describes a method and process for immobilizing radioactive species within a waste material, through the addition of Portland cement in combination with an iron complexant compound, to form a solid material which is then allowed to cure and solidify. U.S. Pat. No. 4,853,208 describes a method of detoxifying hazardous wastes by mixing the waste with silicate and a setting agent such as Portland cement, lime, gypsum or calcium chloride. U.S. Pat. No. 4,354,954 provides a method of solidifying and disposing of aqueous radioactive wastes, whereby the waste is evaporated, the pH is set with the addition of a highly alkaline solution, and the resulting material is spray-dried on a clay-like additive material and pelletized. U.S. Pat. No. 5,545,797 discloses a method of immobilizing plutonium waste with SiO2, the resulting mixture being further cold pressed and then heated under pressure to form the final waste product. U.S. Pat. No. 4,028,265 provides a method of converting sodium nitrate-containing wastes to a solid form by reacting the waste with an aluminosilicate clay at a temperature of from 30° to 100° C., to trap the dissolved radioactive salts in the aluminosilicate matrix. None of the above patents teach immobilization of nitrate or nitrite ions.
Numerous other methods exist for solidifying aqueous waste material. However, these methods consume limited amounts of water in the formation of hydrated compounds which have little or no capacity for incorporating the anionic species dissolved in the aqueous waste into a crystalline solid and which cannot engulf the remaining aqueous waste in the pore structures of the solids which form. None of these methods teach the formation of crystalline compounds whose formation relies on the incorporation of species dissolved in the waste into their crystalline structures.
In the above described methods, large amounts of grouts, e.g. clay, Portland cement, fly ash, or other additive materials, are needed to form a solid material, thus producing a solid waste in large quantities, thus increasing the costs of processing and storage. There also remains a need, therefore, for methods of immobilizing aqueous low level radioactive wastes to provide a smaller volume of solid waste material, to minimize the space required for storage and disposal of such wastes. The extent to which this can be accomplished is related to the molar ratio of moles of water consumed per mole of crystalline solid formed.